Reid Carson wrote:

Brian McGee writes:

Kodachi is, beyond any doubt, my favorite character.  That is one of the
reasons that this fic caught my attention.  This fic was one of the
better representations of her that I have seen.  She is not insane, she
is merely slightly deluded in the same way that Shampoo is.  She
believes that Ranma loves her with every fiber of his being.

     Okay, I like Kodachi too, but this is simply wrong. As she says in the
Asuka story, she knows Ranma doesn't love her - she just feels that since
he's never said he loves anyone, she's still got a chance, and who can
blame her for that. I do agree that, based on the manga, she doesn't
appear to be insane - in fact, she's got a better grasp on reality than
her brother. She's just got an irritating mannerism (her laugh) and
some unfortunate habits, which aren't really any worse than some of
the things Shampoo, Ukyo, or even Ryoga have done.

Insanity is a legal term, not a medical or scientific one.  An "insane"
person is one who is incapable of determining right from wrong, or who
is unable to restrain him or herself from committing a crime.  Although
we might consider a psychopath to be "crazy," psychopaths are not judged
insane; they are aware of the difference between right and wrong and
feel no particular compulsion to commit crimes.  

Mental illness, on the other hand, covers a wide variety of disorders,
some of which cause dementia -- what we usually think of as "insanity"
-- and some of which do not.  Futhermore, detachment from reality can
range from being so subtle as to be unnoticable to being utterly
disabling.  You can't simply class people into the categories of
"totally sane" and "completely nutty" without having a huge leftover of
disturbed-but-functional people.  Remember that Ranma himself has a
severe mental illness, and yet he normally functions just fine.

Finally, "mannerisms" and "habits" -- i.e. behavior -- are where all
mental illnesses manifest themselves.  This is part of what makes
diagnosing them so difficult.  For example, take two people who won't
drink coffee.  Person A thinks coffee is disgusting, while person B
thinks that Juan Valdez is trying to kill him by poisoning his coffee. 
One "sane" person, one "crazy" one... but all a casual observer can tell
is that neither one drinks coffee.  

-- David